Helmet plenum



FIG. 1 is a back side perspective view of a helmet plenum showing my new design;

FIG. 2 is a top side view thereof;

FIG. 3 is a front side view thereof;

FIG. 4 is a back side view thereof;

FIG. 5 is a bottom side view thereof;

FIG. 6 is a right side view thereof; and,

FIG. 7 is a left side view thereof.

The broken lines depict portions of the helmet plenum that form no part of the claimed design. 

CLAIM The ornamental design for a helmet plenum, as shown and described. 